The present invention relates generally to hydraulically operated elevators which are raised and lowered by an hydraulic ram assembly, and is particularly concerned with a control valve assembly for controlling the movement of such elevators.
Control valves for hydraulically operated elevators control the flow of hydraulic fluid into and out of the elevator cylinder in order to raise and lower the elevator. Typically, fluid is supplied to the cylinder from a pump in order to raise the elevator and the cylinder is connected to a reservoir to lower the elevator, the weight of the elevator car forcing fluid out of the cylinder when the cylinder is connected to the reservoir. However, if the pressure in the elevator cylinder is allowed to change suddenly, the resultant sudden movement of the elevator car will be uncomfortable to passengers. Similarly, sudden cut off of fluid supply to or from the cylinder will cause a sudden, jarring stop of the elevator car. Additionally, speed of movement of the elevator car will be dependent on the weight of passengers in the car if a simple on-off control valve is used.
In the past, complex control valves have been devised in attempting to overcome these problems.